1. Field of the Invention
The present invention relates to a preloader for use in a webbing retractor and adapted to allow a webbing to be applied to a vehicle occupant with appropriate tightness at the time of a sudden deceleration of a vehicle.
2. Description of the Related Art
A seat belt apparatus ensures the safety of an occupant by restraining the occupant with appropriate tightness by means of a webbing at the time of a sudden deceleration of a vehicle.
For this reason, preloaders have been proposed for allowing a webbing to be applied closely around the occupant by forcibly pulling the webbing in the direction of retraction.
These preloaders are so arranged as to allow the webbing to be retracted and paid out freely without hampering the rotation of a takeup shaft, during the normal running of the vehicle. At the time of a sudden deceleration of the vehicle, however, the retracting torque of the preloader is transmitted to the takeup shaft by means of a clutch mechanism interposed between the webbing takeup shaft and the preloader, thereby allowing the webbing to be taken up in the direction in which the occupant is restrained.
As for the structure of such a clutch mechanism, the following structures are conventionally known: one in which a latch member (pawl or the like) is meshed with a ratchet gear so as to transmit the retracting torque of the preloader to the takeup shaft (Japanese Patent Application Laid-Open No. 57-128169 etc.), and another in which a wire is wound with play around a rotating drum provided integrally with the takeup shaft, and the rotating drum is rotated by means of a frictional force occurring during tight winding so as to transmit the retracting torque of the preloader to the takeup shaft (Japanese Patent Application Laid-Open No. 59-95068 etc.).
With the clutch mechanism using the latch member (pawl or the like) and the ratchet gear, however, there is a drawback in that in the case of a large torque acting on the latch member, there are instances in which a portion of the latch member engaging with the ratchet gear becomes broken. Meanwhile, with the mechanism in which the rotating drum is rotated by means of frictional force occurring during tight winding, there is a drawback in that the wire can loosen and slip over the peripheral surface of the rotating drum, making it difficult to impart sufficient torque to the rotating drum.